Delegate declaration and instantiation in C Sharp (C#) - Mini Project: Build & Apply

Delegate declaration and instantiation

📖 Scenario: You are creating a simple program to greet people using different greeting styles. You will use a delegate to hold references to greeting methods.

🎯 Goal: Build a program that declares a delegate, creates a delegate instance pointing to a greeting method, and calls it to display a greeting message.

📋 What You'll Learn

Declare a delegate type named GreetDelegate that takes a string parameter and returns void.

Create a method named SayHello that takes a string parameter and prints a greeting message.

Instantiate a GreetDelegate delegate named greet pointing to the SayHello method.

Invoke the greet delegate with the name "Alice".

Print the greeting message to the console.

💡 Why This Matters

🌍 Real World

Delegates are used in event handling, callbacks, and designing flexible APIs where methods can be passed around as variables.

💼 Career

Understanding delegates is essential for C# developers working with GUI applications, asynchronous programming, and frameworks like .NET.

1

Declare the delegate type

Declare a delegate named GreetDelegate that takes a string parameter and returns void.

C Sharp (C#)

// Declare the delegate type here
// Your code here

Hint

A delegate declaration starts with the keyword delegate, followed by the return type, delegate name, and parameter list.

2

Create the greeting method

Create a method named SayHello that takes a string parameter called name and prints "Hello, {name}!" to the console.

C Sharp (C#)

public delegate void GreetDelegate(string name);

// Create the SayHello method here
// Your code here

Hint

Use System.Console.WriteLine with an interpolated string to print the greeting.

3

Instantiate the delegate

Inside the Program class, create a GreetDelegate variable named greet and assign it to the SayHello method.

C Sharp (C#)

public delegate void GreetDelegate(string name);

public class Program
{
    public static void SayHello(string name)
    {
        System.Console.WriteLine($"Hello, {name}!");
    }

    // Instantiate the delegate here
    // Your code here
}

Hint

Assign the method name SayHello directly to the delegate variable.

4

Call the delegate and print the greeting

In the Main method, call the greet delegate with the argument "Alice" to print the greeting message.

C Sharp (C#)

public delegate void GreetDelegate(string name);

public class Program
{
    public static void SayHello(string name)
    {
        System.Console.WriteLine($"Hello, {name}!");
    }

    public static void Main()
    {
        GreetDelegate greet = SayHello;
        // Call the delegate with "Alice" here
        // Your code here
    }
}

Hint

Invoke the delegate like a method with the string argument.

Practice

1. What is a delegate in C# used for?

easy

A. To define variables that cannot change

B. To hold references to methods with a specific signature

C. To store values like integers or strings

D. To create new classes dynamically

Solution

1. Step 1: Understand delegate purpose

   A delegate is a type that holds references to methods with a matching signature.

2. Step 2: Compare options to definition

   Only To hold references to methods with a specific signature correctly describes this purpose; others describe unrelated concepts.

3. Final Answer:

   To hold references to methods with a specific signature -> Option B

4. Quick Check:

   Delegate = method reference holder [OK]

Hint: Delegates point to methods, not data or classes [OK]

Common Mistakes:

• Confusing delegates with variables
• Thinking delegates create classes
• Assuming delegates store simple values

2. Which of the following is the correct way to declare a delegate that takes an int and returns void?

easy

A. void delegate MyDelegate(int number);

B. delegate int MyDelegate(void number);

C. delegate MyDelegate void(int number);

D. delegate void MyDelegate(int number);

Solution

1. Step 1: Recall delegate declaration syntax

   The correct syntax is: delegate returnType DelegateName(parameterList);

2. Step 2: Match syntax to options

   delegate void MyDelegate(int number); matches this exactly: delegate void MyDelegate(int number); others have incorrect order or types.

3. Final Answer:

   delegate void MyDelegate(int number); -> Option D

4. Quick Check:

   delegate returnType Name(params) = delegate void MyDelegate(int number); [OK]

Hint: Remember: 'delegate' keyword + return type + name + parameters [OK]

Common Mistakes:

• Placing 'void' before 'delegate'
• Swapping delegate name and return type
• Using 'void' as parameter type

3. What will be the output of the following code?

delegate int Operation(int x, int y);
class Program {
  static int Add(int a, int b) => a + b;
  static void Main() {
    Operation op = Add;
    Console.WriteLine(op(3, 4));
  }
}

medium

A. 0

B. 34

C. 7

D. Compile error

Solution

1. Step 1: Understand delegate assignment

   The delegate 'Operation' matches the method 'Add' signature (two ints, returns int), so 'op' points to 'Add'.

2. Step 2: Evaluate delegate invocation

   Calling 'op(3, 4)' calls 'Add(3, 4)', which returns 3 + 4 = 7.

3. Final Answer:

   7 -> Option C

4. Quick Check:

   Delegate calls Add(3,4) = 7 [OK]

Hint: Delegate calls assigned method with given arguments [OK]

Common Mistakes:

• Thinking delegate concatenates numbers
• Expecting string output
• Assuming compile error due to delegate

4. Identify the error in the following delegate instantiation:

delegate void Printer(string message);
class Program {
  static void PrintMessage() { Console.WriteLine("Hello"); }
  static void Main() {
    Printer p = PrintMessage;
    p("Hi");
  }
}

medium

A. PrintMessage method signature does not match delegate

B. Delegate cannot be assigned to a method

C. Missing parentheses when assigning delegate

D. Delegate must return a value

Solution

1. Step 1: Compare delegate and method signatures

   The delegate 'Printer' expects a method with a string parameter and void return type.

2. Step 2: Check method signature

   'PrintMessage' has no parameters, so it does not match the delegate signature.

3. Final Answer:

   PrintMessage method signature does not match delegate -> Option A

4. Quick Check:

   Delegate and method signatures must match [OK]

Hint: Delegate and method signatures must match exactly [OK]

Common Mistakes:

• Ignoring parameter mismatch
• Thinking delegate needs parentheses on assignment
• Assuming delegate must return a value

5. Given the delegate declaration:

delegate bool Filter(int value);

Which of the following correctly instantiates the delegate to a method that returns true if the number is even?

hard

A. Filter f = x => x % 2 == 0;

B. Filter f = (int x) => { return x * 2; };

C. Filter f = delegate(int x) { Console.WriteLine(x); };

D. Filter f = x => x + 2 == 0;

Solution

1. Step 1: Understand delegate signature

   'Filter' expects a method taking an int and returning a bool.

2. Step 2: Check each option's return type and logic

   Filter f = x => x % 2 == 0; returns true if x is even (x % 2 == 0), matching bool return.
   Filter f = (int x) => { return x * 2; }; returns int, not bool.
   Filter f = delegate(int x) { Console.WriteLine(x); }; returns void (no return), invalid.
   Filter f = x => x + 2 == 0; returns bool but checks x + 2 == 0, which is not the even check.

3. Final Answer:

   Filter f = x => x % 2 == 0; -> Option A

4. Quick Check:

   Delegate needs bool return matching even check [OK]

Hint: Match delegate return type and logic exactly [OK]

Common Mistakes:

• Returning wrong type (int instead of bool)
• Missing return statement in anonymous method
• Incorrect condition for even check